In general, a Zener diode operates with a reverse bias voltage when the Zener diode is included in a circuit. When this reverse bias voltage applied to the Zener diode exceeds a certain value, the current flowing through the diode rises rapidly due to the electron tunneling effect. This reverse bias voltage is referred to as the Zener breakdown voltage. The current flow in the forward direction of a Zener diode is similar to that of a traditional diode.